Rotors for devices such as compressors, pumps and rotary engines are finished to precise dimensions and tight tolerances. Rotors are thus among the more expensive and time consuming components in the manufacture of such devices. Rotors are furthermore complex parts because they receive one or more reciprocating vanes in slots that pass through the rotor body. It is a challenge to manufacture rotors economically and rapidly by conventional machining techniques, especially if it is desired that the rotor be formed from a single piece of metal. There is thus an opportunity to improve the design of rotors to promote their more efficient and rapid manufacture.